Color processing is a difficult task, especially when transforming (such as e.g. to obtain a color specification directly usable for driving a display, whereby the display can take care of handling its specifics, like e.g. a display dependent electro-optical rendering behavior or gamma) colors between color representations corresponding to an input and output luminance dynamic range which differ considerably, e.g. the largest one is at least 1.5 times larger than the smaller one. This can happen e.g. when transforming from High dynamic range (HDR) to Low dynamic range (LDR), or vice versa. HDR may refer to rendering on a display with higher peak brightness, and typically also deeper blacks, but also signal encoding with a higher captured scene dynamic range, or to be used with such a higher dynamic range display. The skilled reader understands if we say the representation corresponds to a particular dynamic range, that it is specified according to that range. A color specification means not so much (especially in the technical field of HDR encoding or HDR rendering) unless we specify which luminances correspond to the color coordinates (e.g. should one show a photo of a tapestry with the maximum brightness, or with a reduced brightness so that it looks more like a tapestry and less like a luminous object). E.g., in a relative RGB encoding, we may say that the white [1,1,1] corresponds with a maximum displayable peak brightness of 2000 nit (for a HDR display), and e.g. 2× less for an other (“LDR”) display for which we will derive a new color specification. If the color is specified device dependently, the maximum of the dynamic range may be the actual display peak brightness, and for generic color encodings, one can consider the maximum encoding (e.g. 255,255,255) corresponding with a reference display peak brightness (e.g. 5000 nit). This principle can be applied in whatever way one encodes the color (e.g. in non-linear Lab, etc.).
Now prior art attempts to map from e.g. HDR to LDR have seen considerable difficulties in obtaining good (e.g. natural looking, or with the output/LDR colors reasonably matching the input HDR/colors at least in their chromatic appearance not their brightness or lightness) colors as output. At least three reasons contribute to this. Firstly, several color spaces used in practice are non-linear in an inconvenient way, and even then people don't always use them in the correct way, which leads to (sometimes difficult to predict) color errors, which then need considerable further correction. Secondly, different displays have their own inherent limitations, and an LDR display just cannot make as many bright colors as an HDR display. This may e.g. lead to choices in the top of the LDR gamut to make colors less saturated. Lastly, ultimately the quality of the colors is judged by the appearances to the human viewer, and this is also a highly complex process. A big mathematical issue when inappropriately mapping colors is already the clipping which occurs when mapping colors to non-realizable, out-of-gamut colors for the final display and/or output color space.
An example of a system which adapts an image for displaying on a display with a different dynamic range is provided in US2006/0104508. In the system, specular highlights are detected in an LDR image and the additional dynamic range of an HDR display is used to increase the brightness of the specular highlight. The system achieves this by applying a piecewise linear transformation.
Prior art like R. Mantiuk et al. “Color Correction for Tone Mapping,” Eurographics Vol. 28 (2009), pp. 193-202, start by specifying a tone mapping in the luminance direction, and then apply this tone mapping on the luminances of the pixels in the input image. The then remaining color problems are handled, but it is still difficult to get good colors.
An object of our invention is to also have a color processing which can handle inter alia the mapping between a color representation for a higher and lower luminance dynamic range, which takes particular care of handling the chromatic appearance of the colors correctly. And preferably also we have a simple method, which can cheaply be incorporated in many apparatuses.